Power line communication (PLC) is an attractive technology that has received a lot of attention from the research community in the last years. Since power lines were not originally developed for communication purposes, power line communications generally take place in noisy environments that seriously affect the data transfer between two points of the PLC network. The literature has classified the noise present on the power line channel into two categories: asynchronous and synchronous with the alternating current (AC) power line. Synchronous noise is generated by devices having impulsive power absorption synchronous with the main voltage.
In the literature (e.g. see the article A. Lasciandare, S. Garotta, F. Veroni, E. Saccani, L. Guerrieri and D. Arrigo, “Experimental field trials of a utility AMR power line communication system analyzing channel effects and error correction methods,” IEEE ISPLC 2007, pp. 144-149, March. 2007), the low voltage (LV) power line network, the last distribution level of the electric distribution line, has been considered and extensive measurements have been done in the ENEL simulated field with the aim to characterize the power line medium for automated meter reading (AMR) applications. In this framework, the power line channel was used for the communication between the data concentrator (acting as a master) and the electricity meters (the slaves). The results presented in the article pointed out that, at the considered frequencies, the most critical noise, which dominates over the other types of noise, is the synchronous noise. Synchronous noise generators include, but are not limited to, switched-mode power supplies (SMPS), lamp ballasts and power factor correction (PFC) units.
In FIG. 1, FIG. 2 and FIG. 3, the time relationship among phase 1 (continuous line), phase 2 (point-dotted line) and phase 3 (dotted line) of a 50 Hz three-phase system is shown together with the typical noise distributions highlighted in the above mentioned article. In particular, FIG. 1 features the case of a noise that is synchronous with phase 1; FIG. 2 and FIG. 3 consider the most detrimental cases of a noise synchronous with more than one phase voltage.
In FIG. 1, the noise is concentrated in correspondence with the peaks of the phase 1 sinusoidal wave and noise intensity is represented via a Gaussian distribution, which is typically observed in the applications addressed by the present invention. In FIG. 2, the noise is synchronous with phase 1 and phase 2. The noise pulses synchronous with the peaks of phase 1 have a greater intensity than noise pulses synchronous with phase 2, which are, however, significant. Clearly, the other situation in which the noise synchronous with the peaks of phase 2 is greater than the phase 1 synchronous noise is also possible. FIG. 3 illustrates an example of the worst case in which the noise is synchronous with all the three phases.
To obtain good communications in noisy conditions, generally redundancy is introduced at the transmitter side to protect the useful data information. A further protection, in particular against burst noise, is achieved by interleaving the useful data in transmission. If both coding and interleaving are employed, the receiver, via a de-interleaving, spreads the errors that have occurred in bursts and uses decoding to correct the isolated errors, thus considerably improving the robustness of the communication.
Nevertheless, very often coding and interleaving are not sufficient to obtain reasonable performances. In particular, in noisy conditions such as those reported in FIG. 1, FIG. 2 and FIG. 3 and, especially, if the noise is synchronous with more than one phase, the interleaver effectiveness diminishes because the maximum possible achievable error separation is strongly reduced. As a consequence, also the error correction capability of the code is not sufficient to handle the de-interleaved data to reconstruct the original transmitted information. Moreover, for applications like AMR, the requirements of very small chip sizes may not allow the use of the most powerful coding methods, like for instance turbo coding, because they may heavily impact on the whole chip area.
The skilled artisan will notice that figures similar to FIG. 1, FIG. 2 and FIG. 3 may be generated also for a 60 Hz three-phase system in use for example in the USA. Moreover, in FIG. 1, FIG. 2 and FIG. 3 a three-phase system, with the three phases mutually out of phase by 2π/3 radians, is depicted. Clearly, similar situations may be represented also for other poly-phase systems like two-phase systems with the two phases out of phase by π radians or for a single-phase system.